Drop on demand inkjet technology for producing printed media has been employed in commercial products such as printers, multifunction products, plotters, and facsimile machines. Generally, an inkjet image is formed by selectively ejecting ink drops from a plurality of drop generators or inkjets, which are arranged in a printhead or a printhead assembly, onto an image substrate. For example, the printhead assembly and the image substrate are moved relative to one other and the inkjets are controlled to emit ink drops at appropriate times. The timing of the inkjet activation is performed by a printhead controller, which generates firing signals that activate the inkjets to eject ink. The image substrate may be an intermediate image member, such as a print drum or belt, from which the ink image is later transferred to a print medium, such as paper. The image substrate may also be a moving web of print medium or sheets of a print medium onto which the ink drops are directly ejected. The ink ejected from the inkjets may be liquid ink, such as aqueous, solvent, oil based, UV curable ink or the like, which is stored in containers installed in the printer. Alternatively, the ink may be loaded in a solid form that is delivered to a melting device, which heats the solid ink to its melting temperature to generate liquid ink that is supplied to a print head.
The magnitude, frequency, shape, and other aspects of the firing signals to the inkjets affect the amount of ink ejected from an inkjet. Manufacturing tolerances in printer production processes, however, cause inkjets to respond differently to the same firing signals. Additionally, manufacturing tolerances produce inkjets that eject ink in slightly different directions. Consequently, the landing position for an ink drop ejected from an inkjet nozzle may be different than nominally expected. During manufacture and in user facilities, test patterns may be printed on media or another type of imaging member, such as a drum or belt, and an optical sensor array may be used to sense a test pattern of ink drops. The test pattern provides information about some physical parameter of the ink drops, such as the presence, position, size, shape, and/or color of the ink drops.
Optical sensor arrays are used in printers to measure characteristics of ink ejected by inkjets of a print head. The sensor array is mounted with reference to a light source and an image substrate. The light source includes one or more light emitting diodes (LED) and optics for directing light. The light is directed towards the image substrate, which typically is media, a drum, or a belt. The sensor array is positioned to receive the light specularly, diffusely, or both reflected by the image substrate. The reflectance measurements may provide information about the presence, position, size, shape, color, other characteristics of the ink on the substrate. These measurements may be used to adjust driving signals to the nozzles or other imaging parameters and corrective actions.
A sensor array for detecting light reflected by an image substrate is typically used to measure the intensity of light reflected from a surface bearing an ink image. Such an array may be comprised of a plurality of charge coupled devices (CCDs) or contact image sensors (CISs) that are aligned on a support structure facing an image substrate. Other surface imaging systems include those that utilize various types of light collectors, such as prisms, light pipes, microscopic or other refractive lens systems. In some printers, a sensor array may include 600 sensing elements (pixels) per linear inch. Consequently, an 8.5 inch wide sensor array would have 5100 sensing elements. The sensing elements may respond differently to the same ink on the substrate because the characteristics of each photosensitive sensing element may differ. That is, the signal generated by one photosensitive sensing element in the array in response to a reflected light signal may differ from the signal generated by another photosensitive sensing element in the array in response to the same reflected light. Another issue affecting reflectance measurements obtained by sensor arrays is the structure of the image substrate. Many image substrates are rough and highly structured. Some sections of the surface reflect light more intensely into the light sensor, while other sections absorb more light and reflect less light into the sensor.
Another issue affecting reflectance measurement is absence of uniformity across the image substrate in the light source. The amount of light reflected by ink on the substrate and detected by the sensor depends upon the amount of incident light. Registration between a light source, image substrate, and optical sensor may also affect reflectance measurements. The light collected by the sensor is dependent upon the distances and angles between a light source, image substrate, and optical sensor. Variations in these characteristics across the array may arise in manufacturing and/or during a scan of an image substrate. These issues affect the consistency and the clarity of test pattern images obtained by an optical sensor array.
Two types of test patterns are commonly used for obtaining data that may be used to adjust printing parameters for a printer. One type of test pattern, which is called a fiducial pattern, is printed with a predetermined pattern that facilitates detection and identification of the fiducial in the image developed from the electrical signals generated by the optical detectors in a sensor array. Once some or all of the fiducials are located in the image, this information may be used to determine a coordinate transformation between the internal representation of the image and the detected image. The coordinate transformation is used to translate between coordinates for ink drops on the image substrate and coordinates of the internal representation of the image. The other type of test pattern printed on an image substrate is a highly structured pattern that is used to measure one or more physical parameters of the ink drops, such as the presence, position, size, shape, and/or color of the ink drops. These measurement patterns are used to generate adjustment parameters that are used during printer operation to adjust digital images to be printed, to modify firing signals, and/or to modify the timing of firing signals to provide high quality images. In short, the fiducial patterns enable the image processor to identify a coordinate transformation between the external environment as it is captured by the image sensor array and the internal representation of the image that was rendered to produce the ink image. With that information, the image processor is then able to evaluate the more structured measurement patterns for adjustment parameter generation.
As noted above, measurements are affected by a variety of variations, including structure in an image substrate and/or image sensor system. Additionally, printers may include defective inkjets, some of which may have been fired unsuccessfully during generation of a fiducial pattern. Also, fiducial patterns and measurement patterns may be printed in close proximity to one another. Thus, measurement variations, the structure of the image substrate, the possibility of missing ink drops in a fiducial pattern, and the close proximity of fiducial and measurement patterns may make identification of the fiducial patterns by the image processor difficult. Moreover, the image processor needs to be able to locate correctly the fiducial patterns and to distinguish the fiducial patterns from the measurement patterns.